1. Field of the Invention
The present invention relates generally to the field of electrochemical cells, and more particularly to the method of forming an electrically conductive interconnection layer on an electrode of a high temperature, solid oxide electrolyte, electrochemical cell by plasma spraying a compensated plasma spray feed powder to account for differential volatilization during plasma spraying. Even more particularly, the invention relates to a plasma or thermal sprayed dense, substantially leak proof, substantially hydration-free, electrically conductive doped lanthanum chromite interconnection layer bonded to an electrode of a high temperature, solid oxide electrolyte, electrochemical cell to serve as an electrical connection to an electrode of a second electrochemical cell.
High temperature, solid oxide electrolyte, electrochemical generator devices are taught in U.S. Pat. Nos. 4,345,468 (Isenberg) and 4,490,444 (Isenberg). These types of electrochemical generator devices are made up of a plurality of elongated, typically annular, electrochemical cells or fuel cells, which operationally convert chemical energy into direct-current electrical energy. The fuel cells are interconnected in series to provide a desired voltage and in parallel to provide a desired system current.
Each fuel cell typically includes an, optional, porous support tube, typically made of calcia stabilized zirconia which has a porous, annular, air electrode or cathode deposited generally surrounding the support tube. The air electrode is typically made of doped oxides of the perovskite family, such as LaMnO.sub.3. Substantially surrounding the major portion of the air electrode is a dense layer of gas-tight solid electrolyte, typically made of yttria stabilized zirconia. Substantially surrounding the solid electrolyte is a porous, fuel electrode or anode, typically made of nickel-zirconia cermet or cobalt-zirconia cermet. Both the solid electrolyte and the outer electrode, or, in this instance, the fuel electrode, are discontinuous at a selected radial segment to allow for the inclusion of an electrically conductive, gas-tight, dense interconnection material which serves as an electronic interconnection between adjacent fuel cells. A selected radial segment of the air electrode is, accordingly, covered by the interconnection material. The interconnection material is typically made of doped lanthanum chromite film. The dopant typically used is Mg, although other suggested dopants are Ca and Sr. The dopant substituted on the La.sup.3+ and/or Cr.sup.3+ sites serves, inter alia., to enhance the conductivity of the lanthanum chromite p-type conductor.
Various methods have been used to apply the interconnection material on top of the air electrode. U.S. Pat. Nos. 4,597,170 (Isenberg) and 4,609,562 (Isenberg, et al. ) teach the conventional application of an interconnection material on the surface of a selected portion of an air electrode using a modified electrochemical vapor deposition process, at temperatures of about 1300.degree. C. to 1450.degree. C., in a reducing atmosphere, with the suggested use of vaporized halides of lanthanum, chromium, magnesium, calcium or strontium for the deposition of the interconnection material onto the air electrode.
It has been found, however, that there are certain thermodynamic and kinetic limitations in doping the interconnection material from a vapor phase by a vapor deposition process at temperatures between about 1300.degree. C. to 1450.degree. C. The vapor pressures of, for example, the calcium chloride and strontium chloride are low at vapor deposition temperatures, and therefore, the transport of these dopants to the reaction zone at the surface of the air electrode is difficult. Thus, magnesium has conventionally been used as the primary dopant for the lanthanum chromite interconnection material. However, a magnesium doped lanthanum chromite interconnection has a substantial thermal expansion mismatch with the air electrode and electrolyte materials which can cause destabilization effects and reduce electrochemical generator output. Additionally, the use of halide vapors at 1300.degree. C. to 1450.degree. C. in a reducing atmosphere can interact with the air electrode material during the initial period of interconnection deposition. This can cause air electrode leaching of its constituents, such as Mn, into the interconnection which also can cause destabilization effects and problems in the electrochemical generator output. Also the long term stability of interconnection layers made by electrochemical vapor deposition techniques remains questionable even though such techniques initially form substantially leak tight interconnections.
U.S. Pat. No. 4,598,467 (Ruka), in an attempt to solve the interconnection thermal expansion mismatch problems, taught cobalt doped lanthanum chromite, preferably also doped with magnesium, for example, LaCr.sub.0.93 Mg.sub.0.03 Co.sub.0.04 O.sub.3, also by a modified electrochemical vaporization method using chloride vapors of lanthanum, chromium, magnesium and cobalt. However, cobalt chloride vapors similarly have low pressures at vapor deposition temperatures, and accordingly, suffer from similar thermodynamic and kinetic limitations which results in an inadequately doped interconnection.
Other methods of making doped lanthanum chromite interconnection materials have been tried. U.S. Pat. No. 4,861,345 (Bowker, et al.), in a different approach for forming the interconnection layer on an air electrode of a electrochemical cell, taught solid-state sintering of deposited particles of LaCrO.sub.3 doped with Sr, Mg, Ca, Ba or Co coated with calcium oxide and/or chromium oxide. The calcium oxide and chromium oxide coating lowered the sintering temperature from about 1700.degree. C. to between about 1300.degree. C. to 1550.degree. C. without substantial loss of volatile Cr and/or chromium oxides from the interconnection material, and provided greater inclusion of Ca dopant into the interconnection material, and also provided a 95% densified, doped gas-tight interconnection material. However, high temperature sintering of the doped LaCrO.sub.3 particles coated with CaO and Cr.sub.2 O.sub.3 to produce an interconnection material on the air electrode can result in problems. These problems include Mn leaching from the air electrode into the interconnection material and diffusion of the interconnection material dopant into the air electrode.
U.S. Pat. No. 4,895,576 (Pal, et al.), in another approach, taught forming a layer of metal oxide particles selected from the group oxides of Ca, Sr, Co, Ba and Mg, for example, CaO, CaO.sub.2, SrO, SrO.sub.2, CoO, Co.sub.2 O.sub.3, BaO, BaO.sub.2, MgO, or MgO.sub.2, on the interconnection portion of a fuel cell air electrode, heating the air electrode to about 1300.degree. C. to 1450.degree. C., modified electrochemical vapor depositing a skeletal structure of lanthanum chromite around and between the metal oxide particles using halide vapors of lanthanum and chromium, and annealing at temperatures between about 1100.degree. C. to 1400.degree. C. to further cause the metal ions of the metal oxide particles to diffuse in the bulk of lanthanum chromite interconnection material to provide an electrically conductive interconnection. This process requires an additional long term annealing step to maximize conductivity. Some of the dopant can also diffuse into the air electrode at such high temperatures.
None of these techniques, however, solve all the potential problems of thermal expansion mismatch, Mn leaching from the air electrode, dopant leaching from the interconnection material, and problems associated with doping Ca, Sr, Co, Ba and other materials by vapor deposition or of providing a uniformly thick, dense, leak tight, well-bonded, electrically conductive interconnection layer on a variety of substrates in a simple and economical fashion. Many of these problems, including doping and leak tightness, appear to be dictated by the process used to from the interconnection material. There is a need for a method of forming an interconnection layer on an electrode of an electrochemical cell which provides a highly doped, substantially gas-tight, electrically conductive interconnection material without the associated problems.
Plasma arc spraying and flame spraying, i.e., generally known as thermal spraying or plasma spraying, are well-known film deposition techniques. Thermal spraying involves spraying a molten powdered metal or metal oxide onto the surface of a substrate using a thermal or plasma spray gun. U.S. Pat. No. 4,049,841 (Coker, et al.) generally taught plasma and flame spraying techniques. U.S. Pat. Nos. 3,823,302 (Muehlberger) and 3,839,618 (Muehlberger) generally taught plasma spray guns.
Plasma spraying is a technique that has been used for fabrication of some components, other than the interconnection material, in a high temperature, solid oxide electrolyte, electrochemical cell. Plasma spraying, however, has not been effectively used for the fabrication of the interconnection material. Attempts to plasma spray an interconnection material on an electrode structure of an electrochemical cell have been found to remain porous after cell processing, and, further, to lead to intermixing of the fuel and oxidant during cell operation. Gas can pass through such a structure, whereas prior electrochemical vapor deposition and sintering techniques provided a closed pore structure. This intermixing of fuel and oxidant resulting from leakage through the interconnection microstructure is detrimental to electrical power generation, and also to the stability of the cell. Other attempts to plasma spray an interconnection material on an electrode structure of an electrochemical cell have been found to deposit an interconnection material that is not stoichiometric in distinct regions, and further lead to hydration and cracking in these regions. Accordingly, the efficiency and life expectancy of the electrochemical cell are substantially reduced. It is, therefore, desirable for long term electrochemical cell operation and component stability to provide a substantially leak tight, substantially hydration-free, as well as a substantially electrically conductive interconnection.
None of the references teach or suggest effectively plasma spraying an interconnection material, particularly plasma spraying a compensated interconnection powder, and providing a plasma sprayed interconnection that is substantially leak tight, hydration-free and electrically conductive. There is a need to provide a dense, substantially gas-tight, highly doped, electrically conductive interconnection material for use in an electrochemical cell generator. There is a further need to provide an interconnection material that is substantially free of pores, at least at a surface thereof, and also substantially doped by a plasma spray deposition and densification technique, thereby forming an effective gaseous diffusion barrier and an electrically conductive interconnection. There is also a particular need to effectively compensate the plasma spray interconnection feed powder to avoid problems in the interconnection caused from the differential volatilization of certain constituents of the interconnection composition during plasma spraying.